1. Field of the Invention
This invention relates to biosensors. More particularly, this invention relates to biosensors in which the biological sample is transported to a sample chamber by means of wicking of fluid.
2. Discussion of the Art
A biosensor is a device for measuring the concentration of an analyte in a biological sample. A typical biosensor comprises a support, a reference electrode or a counter electrode or both a reference and a counter electrode disposed on the support, a working electrode disposed on the support, the working electrode spaced apart from the other electrode or electrodes on the support, a covering layer defining an enclosed space over the electrodes, an aperture in the covering layer for receiving a sample, and at least one mesh layer in the enclosed space between the covering layer and the electrodes. The working electrode includes an enzyme capable of catalyzing a reaction involving a substrate for the enzyme and a mediator capable of transferring electrons between the enzyme-catalyzed reaction and the working electrode to create a current related to the activity of the enzyme and related to the concentration of the analyte in the sample. Alternatively, instead of an enzyme, the working electrode can include a substrate capable of catalyzing a reaction involving an enzyme for the substrate and a mediator capable of transferring electrons between the substrate-catalyzed reaction and the working electrode to create a current related to the activity of the substrate and related to the concentration of the analyte in the sample. The purpose of the mesh layer or mesh layers is to define a path for directional flow of the sample from the aperture through the enclosed space towards the electrodes and control the height of the enclosed space above the electrodes. The mesh layers are formed of a woven material and coated with a surfactant. An example of a biosensor is shown in U.S. Pat. No. 5,628,890, incorporated herein by reference.
The test sample is required to be delivered rapidly and uniformly from a sample application zone, i.e., at the aperture, to a reaction zone within the enclosed space, which is referred to herein as a sample chamber. Typically, delivery of the test sample is carried out by wicking along the mesh layer, which is typically of a hydrophilic character for biological samples. See U.S. Pat. No. 5,628,890, EP 0170375, U.S. Pat. No. 5,141,868, and U.S. Pat. No. 6,436,256. Sample chambers of biosensors are preferably constructed so that they have a small volume for the purpose of reducing the amount of test sample (generally blood) required from a patient.
This approach has advantages in that the use of a mesh layer allows one dimension of the sample chamber to be tightly controlled while also reducing the void volume, thereby reducing the volume of the test sample required. Woven mesh layers are generally fabricated from synthetic polymeric fibers of known diameter, typically nylon and polyester fibers. Nylon and polyester fibers are relatively hydrophobic and, consequently, meshes constructed from the untreated fibers are unsuitable for direct use for promoting transportation of the test sample in a biosensor.
U.S. Pat. No. 5,628,890 discloses the use of a surfactant-coated mesh layer in a biosensor for the purpose of wicking. A fluorinated surfactant, “FLUORAD FC-170C” (3M Company, St. Paul, Minn.) is disclosed as a preferred surfactant in this system. Manufacture of the fluorinated surfactant FC-170C was terminated by the 3M Company because of concerns relating to its effect on the environment. Furthermore, the Environmental Protection Agency (EPA) has recently imposed restrictions on the manufacture and use of such surfactants and related substances in the United States. Similar fluorinated surfactants are still available from other manufacturers, but there is a legitimate concern that such materials may be withdrawn from the market in the future.
Accordingly, the surfactant “FLUORAD FC-170” needs to be replaced by an equally effective non-fluorinated surfactant, preferably one that is commercially available. A surfactant must fulfill the following requirements: long-term stability, ease of applying onto the mesh layer, in particular, applying by means of an aqueous solution. The Gower Handbook of Industrial Surfactants lists over 21,000 products.
Textile spin finishes are non-permanent coatings applied to fibers and yarns as emulsions in order to improve lubrication and prevent antistatic build-up during processing (Philip E. Slade, Handbook of Fiber Finish Technology, Marcel Dekker (1998)). Spreading of the spin finish emulsion on the surface of the fiber to achieve a uniform coating is promoted by the addition of surfactants to the formulation. This type of spreading is somewhat analogous to the situation with respect to biosensors, where a test sample of high surface tension, i.e., blood, is applied to a surfactant-coated mesh, where initial wetting occurs followed by subsequent spreading. However, an important difference is that the spin finish emulsion contains the surfactant and is applied to the untreated fiber whereas in the biosensor, the fiber is already coated with surfactant and a test sample (without surfactant) is applied to the coated fiber.
Silicone surfactants are available from a number of manufacturers, such as, for example, Dow Corning, OSi Specialities, Basildon Chemicals, Clariant, and Degussa. These surfactants are often used as additives (minor components) of fiber finishes, which are required during processing. They are added to finish formulations to promote wetting of the fiber with the hydrophobic finish and are not used to increase the hydrophilicity of the finished fiber. The fiber finish is required for lubrication and anti-static properties during processing. The prior art offers no specific guidance as to which surfactants will be effective as spreading agents when applied to mesh in biosensors.
The mechanics of the spreading/wicking process is complex. The coating emulsion requires a low surface tension to wet the surface of the fiber or yarn, but the wicking rate is greater at a high level of surface tension (Philip E. Slade, Handbook of Fiber Finish Technology, Marcel Dekker (1998), pg. 45-48). For example, fluorinated surfactants are known to be among the most effective at lowering surface tension but are reported to have “a considerable negative effect on wicking” (Wicking of Spin Finishes and Related Liquids into Continuous Filament Yarns, Y. K. Kamath, S. B. Hornby, H.-D. Weigmann and M. F. Wilde, Textile Res. J., 1994, 64, 33-40). This finding is confirmed by spreading studies of surfactant solutions on Parafilm (K. P. Ananthapadmanabhan, E. D. Goddard and P. Chandar, Colloids Surf., 1990, 44, 281).
As stated previously, an important property of a biosensor is its long-term stability. The biosensor is required to function without any deterioration in performance for many months after manufacturing. Satisfactory performance requires the sample chamber to fill rapidly and uniformly over the shelf life of the product. Given that the coating of surfactant on the mesh layer is non-permanent and is necessary for adequate filling of the sample chamber, it follows that the surfactant itself must be chemically stable, while not undergoing excessive migration/diffusion from the mesh layer to other surfaces in the biosensor. Some loss of surfactant from the mesh layer to other hydrophobic surfaces (such as printed electrode tracks) is considered beneficial, because these surfaces will become more hydrophilic. Excessive loss will result in an unacceptable deterioration in wicking performance, leading ultimately to a catastrophic failure to fill. Surfactants having high molecular weight, which are either solids or viscous liquids, are expected to be less mobile and therefore more capable of providing durable spreading capability. However, such materials are expected to be less effective as spreading agents than those surfactants having lower molecular weights.
It is important to consider the interaction of the surfactant coated onto the mesh layer with adjacent layers in the biosensor. The surfactant may inhibit adhesion of other layers to the mesh layer. In addition, the mesh layer may be adhered to the electrode substrate by a screen-printed insulating ink, with which the surfactant could interact adversely. For example, the wet ink printed onto the surfactant-coated mesh layer may wick along the fibers, resulting in poor print definition.
It is not a simple case of applying any surfactant (or even specifically the most effective surfactants such as fluorinated surfactants) to a mesh layer of a biosensor to achieve rapid and uniform wicking of the applied test sample. Adequate models of the mechanics and dynamics of the spreading of surfactant solutions remain to be developed, largely because the phenomenon is so complex (Silicone Surfactants, Surfactant Science Series, Vol. 86, ed. Randall M. Hill, Marcel Dekker, 1999, pg. 303-310). Furthermore, there are other critical factors to consider when selecting a surfactant for specific use in a biosensor; many of these factors have not been considered previously in the literature.
In summary, a number of conflicting factors have to be balanced to obtain the optimal selection from an enormous range of commercially available surfactants. These factors include the ability to lower surface tension, coating stability, coating uniformity, stability of the surfactant, migration effects, adhesion inhibition effects, wicking speed, wicking uniformity, toxicity, and printing definition.